This invention relates to an image signal processing method, image signal processing system, storage medium, and image sensing apparatus.
An image signal processing system that processes color image signals executes predetermined color correction for original signals output from an image sensing element to generate a brightness signal and color difference signals, and displays an image represented by the brightness signal and color difference signals on an image display device or stores them in a storage medium.
Various techniques for reducing the capacity of a storage medium required for storing image data or reducing the network load upon transmitting image data in such image signal processing system have been proposed.
FIG. 10 is a block diagram showing an example of a conventional image signal processing system. As shown in FIG. 10, original signals (complementary original signals Ye, Cy, Mg, and Gr in this embodiment, but the same applies to R, G, and B signals) output from an image sensing element 201 are input to a color processing device 202.
The color processing device 202 is comprised of a pre-processing circuit 202a for executing predetermined pre-processings for the input original signals Ye, Cy, Mg, and Gr to generate a pseudo brightness signal (Y′) and color difference signals (u′, v′), and a color processing circuit 202b for executing color correction processings such as white balance correction, γ correction, and the like for color information using the pseudo brightness signal (Y′) and color difference signals (u′, v′) to generate a brightness signal (Y) and color difference signals (u, v).
The brightness signal (Y) and color difference signals (u, v) output from the color processing circuit 202b are input to an information compression device 203. The information compression device 203 compresses information using a given compression technique such as JPEG, MPEG, H.261, or the like. For example, the device 203 performs the DCTs (Discrete Cosine Transforms) for the input signals in units of predetermined pixel blocks. The DCT-transformed brightness signal (Y) and color difference signals (u, v) are quantized to remove their high-frequency components, thus compressing their information sizes.
The compressed image data is input to an encoding device 204, and is encoded. As encoding, various schemes are available. For example, when variable length coding that assigns code lengths in correspondence with the frequency of occurrence of data is used, information can be further compressed. The compressed and encoded image signal is transmitted via a communication line.
On a receiving side, the image signal input via a variable length coding input device 206 is input to an information expansion device 207. The information expansion device 207 performs inverse DCT processing and inverse quantization processing contrary with the information compression device 203 to restore the brightness signal (Y) and color difference signals (u, v).
The restored brightness signal (Y) and color difference signals (u, v) are output to an image display device or storage device 208. In this manner, an image sensed by the image sensing element 201 is displayed or stored in a storage medium.
In FIG. 10, DCT and quantization are performed as information compression processeings. Besides these processings, various other compression schemes may be used. For example, as shown in FIG. 11, a compression scheme by means of vector quantization/dequantization using a code book (to be referred to as a code book compression scheme hereinafter) may be used.
Note that the same reference numerals in FIG. 11 denote the same parts as those in FIG. 10, and a detailed description thereof will be omitted.
In this prior art, a brightness signal (Y) and color difference signals (u, v) output from the color processing device 202 are input to a code book compression device 210. The code book compression device 210 employs an information compression scheme that compares the pattern of each input signal with a plurality of code books (patterns) pre-stored in a code book storage device 211, finds out the most similar pattern, assigns this pattern to input information, and outputs the code number of the pattern.
The code number output from the code book compression device 210 is output to a medium such as a communication line or the like via a code number output device 212.
On the receiving side of the image signal, the code number sent via the medium is received by a code number input device 213, and is supplied to a code book expansion device 214. The code book expansion device 214 reads out a pattern corresponding to the input code number from a code book storage device 215 to reconstruct (decode) the image signal compressed on the transmitting side. The reconstructed image signal is output to an image display device or storage device 208, thus displaying an image or storing the signal in a storage medium.
In the conventional image signal processing system, original signals (e.g., Ye, Cy, Mg, and Gr) output from the image sensing element 201 are input to the color processing device 202, which compresses a brightness signal (Y) and color difference signals (u, v) that have been subjected to various color correction signal processings, regardless of what information compression scheme is used.
In recent years, since the information compression techniques have advanced, no extreme image deterioration occurs in an image signal even after information compression. However, deterioration of image quality due to block noise and high-frequency noise produced upon information compression cannot be avoided. Hence, since the image quality deteriorates depending on the compression ratio with respect to the image signal, it is not preferable to select a high compression ratio to obtain a high-quality image.
However, when a low compression ratio is set in information compression, the size of image data used for data transmission via a line or a storage medium becomes very large.